For the construction of various devices intended for the exchange of heat and/or water vapor between two air streams it is desirable to have a thin inexpensive material which removes moisture from one of the air streams and transmits it to the other air stream. In some devices it is also desirable that heat and moisture be transmitted across the thickness of material such that the heat and water is transmitted from one stream to the other while the air itself is not transmitted.
For example, to improve indoor air quality, there is a great need to increase levels of outdoor air ventilated into buildings. However, it is important to minimize the cost associated with the introduction of outside air. For example, in winter, introducing cold, dry outdoor air to a building increases the heating load and adds to the fuel requirement. In summer, introduction of warm, humid outdoor air adds greatly to the air conditioning costs. Thus, to minimize costs associated with introduction of fresh air, heat exchangers such as rotating wheel exchangers or plate type heat exchangers have been employed to recover or reject, as needed, a portion of the sensible heat from the indoor air stream through exchange with the outdoor air stream. Further, some of the heat exchangers provide for latent heat exchange by incorporating a means to transmit moisture into the heat exchange surface.
One well know design for constructing heat exchangers employs a rotating wheel made of an open honeycomb structure. The open passages of the honeycomb are oriented parallel with the axis of the wheel and the wheel is rotated continuously on its axis. If applied to heat exchange for building ventilation, outside air would be made to pass through one section of the wheel while inside air would be made to pass in the opposite direction through another portion of the air. Since the heat would be absorbed in one section of the wheel and rejected in the other portion of the wheel, heat is effectively transferred from one stream to the other. For example, in the winter, cold air from the outside would pick up heat from the wheel as it passes through while the wheel is re-heated by interior air being exhausted through another portion of the wheel. The reverse would be true in the summer months.
The effectiveness of the heat exchange is greatly enhanced if the material of the wheel is made of desiccant material because, in addition to the exchange of sensible heat from the air, the latent heat of condensing and evaporating moisture from the air is also exchanged, at least to an extent. These wheels can also be employed in many other applications besides buildings.
Heat exchange wheels are usually fabricated to provide a multiplicity of parallel pores or openings such as a honeycomb structure through which air passes. The wheels can be formed from coated material such as aluminum, plastic, and paperboard or desiccant paper having one side corrugated and one side flat. The wheels are commonly formed by winding or stacking the coated material or desiccant paper into the wheel shape to provide air passageways parallel to the axis of the wheel.
Herewithin, adsorption is defined as a process in which water vapors move from a gas phase onto a solid surface. Adsorption is to be distinguished from absorption, a process in which water vapor moves into the bulk of a porous material, such as the absorption of water by a sponge. Sorption is a more general term that includes both adsorption and absorption and is used herein to mean either adsorption or absorption.
Desiccant wheels are typically constructed using solid-state desiccant material with very high surface areas to adsorb water vapor molecules from an air stream. Solid desiccant wheels are characterized by high substructure weights, low moisture adsorption rates, high air pressure drops, high manufacturing costs, high regeneration temperatures, susceptibility to frost formation, performance degradation over time, bacterial buildup, gaseous cross-contamination, and are clogged by particulate matter.
Powerful liquid desiccants, such as lithium chloride, have been applied to asbestos and cellulous substrates with limited success in the past. Lithium chloride will dissolve into a liquid salt solution upon exposure to a high relative humidity air mass. The liquid salt solution can flow off of the wheel substructure rendering the wheel ineffective. In addition, lithium chloride can become entrained in the air stream and cause corrosion to downstream components. In order to overcome this “weeping” problem, a very low practical limit in the amount of salt desiccant impregnated into the wheel is used. For example, this is on the order of maximum of 11-12% wt. of the substrate for LiCl and up to 25-27% wt. of the substrate for LiBr. As a result of this low amount of desiccant, these desiccant wheels must be very large with a high surface area to have any appreciable capacity. For example, a low desiccant/support ratio, on the order of 0.1 for LiCl, necessitates uneconomically large wheels having great masses of substrate material for the desiccant support, which in turn imposes substantial power requirements for rotation, and heat requirements for regeneration. In certain prior art designs, higher LiCl concentrations as indicated eventually lead to desiccant deliquescence.
In certain applications of dehumidifying elements, it is necessary for the dehumidified air to be very dry. For example, a dew point of beneath −40 degree F. is required when drying plastic granules for the manufacture of PET-products, while still lower moisture contents are sometimes required in respect of dehumidifying air in chambers where moisture sensitive products are tested. A rotor that solely contains solid desiccant particles can not reach these states without unreasonably high temperatures and energy consumption by the regeneration process. Thus, there is a need for novel dehumidifying elements that are able to generate still drier air than that which can be achieved with the dehumidifying element based on solid desiccants.
As stated previously, one economical method of fabricating a wheel type heat exchanger is to form a corrugated sheet and then to roll up or lay up layers of the corrugated material to form the wheel. In a typical commercial corrugation machine one piece of paper or membrane is passed through a pair of slotted or gear-shaped heated rollers to impart to it the corrugated wavy shape. This shaped piece is commonly called the medium. This corrugated sheet or medium is then brought into contact with a flat sheet commonly called the liner. The two are bonded together by applying some adhesive to the top portions of the medium sheet and pressing the medium and liner together between a corrugated roller and flat roller. This forms what is know in the industry as a single sided corrugated material.
Corrugated materials such as common corrugated cardboard used for packaging are generally made from relatively impervious paper precursors. If relatively impervious material is used the application of the adhesive and subsequent bonding of the medium to the liner presents no special problem. However, for the construction of desiccant or heat exchange wheel it is desirable to use a highly porous paper as a precursor. When the adhesive is applied and the medium and liner are pressed together there is a tendency for the adhesive to flow through the paper where it will eventually build up on the forming wheels and cause the medium and liner to delaminate from each other upon exiting the machine. Another problem is that the adhesive tends to flow in all directions within the paper causing a reduction in the porosity and therefore a reduction in the ability of the product to absorb liquids.
As an alternate to the rotating wheel design, some heat exchangers are made with a series of parallel plates. Spaces are provided between these plates to allow two different air steams to pass and the manifolds for such plate type heat exchangers are arranged such that the two different air steams pass on alternate sides of the plates. The plates allow for the exchange of heat between the two air steams while preventing the air streams from coming into contact with each other or mixing with each other.
For example, in the case of a ventilation system for a building, the outside air being brought into the building would constitute one of the air streams and the inside air being ejected from the building would constitute the other stream. In the summer, the hot moist outside air being brought in would be cooled to an extent by the cool dry inside air while it is exhausted. In this way at least a portion of the energy required to cool the interior of the building would be recovered.
Traditionally the plates of such heat exchangers are made from metal or some material that is thin and can conduct heat well.
The efficiency of this type of heat exchanger can be greatly increased if, in addition to transferring sensible heat by simple conduction, water can also be transmitted. In this case the latent heat of vaporization of the water may also be at least partially recovered in addition to the sensible heat transferred by thermal conduction.
Attempts to do this have employed very expensive and specialized polymeric membranes, but have not as yet enjoyed wide spread practical use.
Although the above description has focused upon heat and moisture exchange for buildings, it should be apparent that the same principles may be applied to many other applications as well, such as: heat and moisture exchange for commercial, industrial and food processing; desiccant applications for drying of process air; water recovery from air; and the humidification of air and fuel for fuel cells by recovery of water from the exhaust of the same.
Thus, a need exists to create a material for improving wheel type heat exchangers and plate type heat exchangers.
In the case of a wheel type exchanger, the material should adsorb and/or absorb a large quantity of moisture from the air and subsequently release this moisture when heated or exposed to less humid air. It must be inexpensive, strong, light in weight, resistant to bacterial growth and be readily corrugated or otherwise formed into a honeycomb shape which allows air to pass through while maintaining a low pressure drop. In the case of plate type exchangers, the material should adsorb and/or absorb large quantities of moisture from the air, transmit the moisture through from one face to the other while not allowing the air itself to pass through. It must be formable in thin strong lightweight sheets, be inexpensive, and be resistant to bacterial growth.